The present invention relates to electrically commutated motors operated according to unipolar motor and driver configurations, and more particularly to methods and the devices directed to a less costly and more effective determination of motor position and speed.
Electrically commutated motors have replaced traditional motors in many applications due to a variety of advantages including improved reliability, higher efficiency, lower costs and longer useful life. The most common type of these motors uses three phases, each phase Embodied by a stator winding or group of stator windings. The motor is driven through selective application of drive voltages to the different phases in a repeating sequence known as a commutation cycle.
Electrically commutated motors are generally of two types: sinusoidally commutated motors and trapezoidally commutated motors. In sinusoidally commutated motors, the phases typically are driven simultaneously at different voltages that vary substantially sinusoidally. These motors exhibit BEMF (back electromotive force) wave forms similar to sine waves. Separate position sensors, e.g. Hall effect sensors, are used to generate the rotor position information needed to drive the motor.
In trapezoidally commutated motors, also called square wave driven motors, the phases are driven intermittently. More particularly, in a three phase motor with a bipolar driver configuration, the phases are driven in pairs so that at any given time, one of the phases is not driven. This allows use of the BEMF signal, in particular its zero crossings, to determine rotor positions, a configuration known as sensorless drive since no Hall effect sensors or other position sensors are required. The BEMF signal may be sinusoidal or trapezoidal.
An alternative trapezoidal drive arrangement, known as a unipolar or halfwave motor and drive configuration, requires fewer components than a bipolar configuration since each phase uses one switching element rather than two. Only one of the phases is driven at any given time, so that over a commutation cycle, different pairs of the phases are not driven and produce respective BEMF signals. However, unlike the BEMF signal of the sole undriven phase in bipolar arrangements, neither BEMF signal in a unipolar arrangement provides the unambiguous zero crossings useful in pinpointing rotor angular or temporal positions. Direct measurement of the BEMF signal while commutating the motor, is not possible.
As a result, the conventional method for determining rotor positions, and on that basis determining when to switch from one commutation step to the next, is to mount three Hall effect sensors on the motor to generate position information.
One attempt to overcome this difficulty, in conjunction with trapezoidally driven three phase unipolar motors with essentially sinusoidal BEMF signals, is disclosed in U.S. Pat. No. 5,017,845 (Carobolante, et al). In this instance, the BEMF voltages in two non-energized phases are measured, and the difference between successive voltage difference measurements is used to control commutation. Specifically, a curve of the BEMF differential is generated, and points where the slope of this curve is zero are said to be about thirty degrees from optimal switch points for the commutation cycle.
Although this system may have utility in connection with sinusoidal BEMF signals, it undesirably requires interruption of excitation of the motor phases. Moreover, it is not suitable for trapezoidal commutation with essentially trapezoidal BEMF waveforms, since reliance on a zero slope does not lend itself to accurate readings with BEMF signals having a trapezoidal shape.
Trapezoidally commutated motors frequently are favored over sinusoidally commutated motors due to their lower cost. However, they entail comparatively large current fluctuations in the stator windings during shifts between the driven and undriven states. These fluctuations, known as commutation current ripple, result in a corresponding torque ripple that is undesirable due to increased motor noise and reduced efficiency. U.S. patent application Ser. No. 11/941,426 (Brown) assigned to the assignee of this application and incorporated herein by reference, addresses this problem in connection with bipolar motor and drive configurations. There remains a need to counteract this problem with unipolar configurations
Therefore, the present invention has several aspects directed to one or more of the following objects:                to provide a unipolar commutation circuit and controller operable to selectively apply different voltages to different motor phases for smother operation of trapezoidally commutated motors;        to provide, in three phase electrically commutated motors, sensorless position and speed detection, regardless of whether the back electromotive force waveforms of the motor phases are trapezoidal or sinusoidal;        to provide a means for using differences between the BEMF signals of two undriven phases, measured at projected points of their expected coincidence, to alter a commutation cycle towards synchronization with motor position and speed; and        to provide, in a brushless DC motor configuration including a unipolar commutation circuit, a process for synchronizing the commutation cycle with motor position and speed without sensing or measuring BEMF signals in the motor stator windings.        